Preparations of particles with particle sizes in the nanometer range (nanoscale particles) are used in many branches of industry. This applies in particular to dispersions which contain particles with magnetic, ferroelectric or piezoelectric properties and which can be heated using magnetic, electrical or electromagnetic alternating fields. Such dispersions are used, for example, for the production of adhesives and sealants which cure under the effect of the heat induced by application of magnetic, electrical or electromagnetic alternating fields or where an existing adhesive bond is dissolved. In many branches of industry, particularly in the metal-processing industry, for example the aircraft industry, the motor industry, in the manufacture of utility vehicles and the associated supplier industries or even in the production of machines and domestic appliances and in the building industry, corresponding adhesives and sealants are being increasingly used to join identical or different, metallic and non-metallic substrates together by adhesion or sealing. This method of joining structural components is increasingly replacing conventional joining techniques, such as riveting, screwing or welding, because bonding/sealing offers a number of technological advantages, for example in regard to possible recycling of the components used.
DE-A 199 23 625 describes a process for the production of redispersible metal oxides or metal hydroxides with a volume-weighted mean crystallite size of 1 to 20 nm which are particularly suitable for so-called magnetic fluids (ferrofluids).
DE-A 199 24 138 describes adhesive compositions which contain nanoscale particles with ferromagnetic, ferrimagnetic, superparamagnetic or piezoelectric properties in the binder system and which are suitable for forming dissolvable adhesive bonds. The adhesive bonds can be heated by electromagnetic radiation to such high temperatures that easy dissolving (debonding) is possible.
WO 01/30932 describes a process for the adhesive separation of adhesive bonds. The adhesive bond comprises a heat-softenable thermoplastic adhesive layer or a heat-cleavable thermoset adhesive layer and a primer layer, the primary layer containing nanoscale particles which can be heated by electromagnetic alternating fields.
WO-A 01/28771 describes a microwave-curable composition containing particles capable of absorbing microwaves with a Curie temperature which is higher than the curing temperature of the composition. The particles capable of absorbing microwaves can be ferrites, for example.
EP-A-0 498 998 describes a process for the microwave heating of a polymer material to a predetermined temperature, the polymer material containing dispersed ferromagnetic particles with a Curie temperature which corresponds to the temperature to be reached by the microwave heating. The particle diameter of the ferromagnetic material is in the range from 1 to 100 nm; the Curie temperature is in the range from 50 to 700° C. The target heating temperature can be, for example, the curing or melting temperature of the polymer material or the temperature required to activate a cleavage reaction.
WO 01/14490 describes a process for bonding substrates with hotmelt adhesives which are used in combination with a microwave-activatable primer.
Unpublished German patent application P 100 37 883.8 describes a process for heating a substrate containing metallic, magnetic, ferrimagnetic, ferromagnetic, antiferromagnetic or superparamagnetic particles with a mean particle size of 1 to 5000 nm, the substrate being exposed to microwave radiation with a frequency of 1 to 300 GHz and, at the same time, to a d.c. magnetic field of which the field strength is at least twice as high as the strength of the earth's magnetic field.
The disadvantage of the nanoparticulate compositions mentioned above lies in the inadequate utilization of the energy introduced. Such systems generally have a high content of dispersed particles of which only a small proportion is capable of absorbing magnetic, electric or electromagnetic radiation of a certain applied frequency and hence contributes towards heating. Besides the high cost of the nanoscale particles used, which makes such compositions uneconomic, other disadvantages, for example in regard to the rheological properties of the systems and the properties of the bond (embrittlement tendency), are generally associated with a high content of dispersed particles.